Adjuvant therapy, also known as adjunct therapy, add-on therapy, and adjuvant care, is therapy that is given in addition to the primary or initial therapy to maximize its effectiveness. The surgeries and complex treatment regimens used in cancer therapy have led the term to be used mainly to describe adjuvant cancer treatments. An example of such adjuvant therapy is the additional treatmentusually given after surgery where all detectable disease has been removed, but where there remains a statistical risk of relapse due to the presence of undetected disease. If known disease is left behind following surgery, then further treatment is not technically adjuvant. An adjuvant agent modifies the effect of another agent, so adjuvant therapy modifies other therapy. The term "adjuvant therapy," derived from the Latin term adjuvāre, meaning "to help," was first coined by Paul Carbone and his team at the National Cancer Institute in 1963. In 1968, the National Surgical Adjuvant Breast and Bowel Project (NSABP) published its B-01 trial results for the first randomized trial that evaluated the effect of an adjuvant alkylating agent in breast cancer. The results indicated that the adjuvant therapy given after the initial radical mastectomy "significantly decreased recurrence rate in pre-menopausal women with four or more positive axillary lymph nodes."

The budding theory of using additional therapies to supplement primary surgery was put into practice by Gianni Bonadonna and his colleagues from the Instituto Tumori in Italy in 1973, where they conducted a randomized trial that demonstrated more favorable survival outcomes that accompanied use of Cyclophosphamide Methotrexate Fluorouracil (CMF) after the initial mastectomy.

In 1976, shortly after Bonadonna's landmark trial, Bernard Fisher at the University of Pittsburgh initiated a similar randomized trial that compared the survival of breast cancer patients treated with radiation after the initial mastectomy to those who only received the surgery. His results, published in 1985, indicated increased disease-free survival for the former group.